Bernot et al, U.S. patent application Ser. No. 08/986,253, filed Dec. 5, 1997, now U.S. Pat. No. 6,058,780 entitled, (as amended), "Capacitive Pressure Sensor Housing Having a Ceramic Base" which is assigned to the assignee of this application, discloses a glass ceramic capsule comprised of a cover and a header also referred to as a base that together define an interior chamber. A capacitive pressure sensor is mounted within the chamber. The header is ceramic while the cover is either ceramic or metallic. The cover and header are sealed together with glass frit.
Typically, the glass frit is made into a paste using a vehicle which is a combination of a solvent and a binder. The paste is then applied onto the surfaces to be bonded by either brushing or ink-jetting. A critical requirement for the vehicle is that it have a low temperature burnout capability. It needs to completely bum out before the glass frit softens (i.e. melts) which occurs at about 350.degree. C. (662.degree. F.). One vehicle currently used is amyl-acetate (a solvent) containing a small amount of nitrocellulose (a binder). This vehicle meets the temperature requirement because the nitrocellulose burns out well below 350.degree. C. (662.degree. F.).
A disadvantage to the brushing or ink-jetting methods is that with these methods it is difficult to control the thickness and uniformity of the glass frit as it is applied. Using a screen-printing method to apply the glass frit paste would provide both performance and cost advantage over these other methods because the screen printing method produces a more uniform frit coating layer with easily controlled thickness and can be economically applied in large scale batch production. However, the current vehicle of amyl-acetate and nitrocellulose is not suitable for a screen-printing method because the evaporation rate of amyl-acetate is too fast creating an unstable paste whose thickness is difficult to control. Further, other commercially available screen-printing vehicles do not burn out completely at temperatures below 350.degree. C. (662.degree. F.).
Accordingly, there is a need for a low-evaporation rate, low temperature burnout vehicle that can be used in the formation of a glass frit paste that can be used in a screen printing process.